How to Test Fuel Injectors—3 Simple Methods

Updated on January 21, 2019
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Automotive fuel injectors.
Automotive fuel injectors. | Source

Testing fuel injectors sometimes can be easier than you think, whether on a TBI (throttle body injection) or an EFI (electronic fuel injection) system.

Often, these injectors face common problems you can diagnose visually (TBI), or with the use of some simple tools.

The modern fuel injection system works more efficiently and reliably than the previous carburetor model. The system has a better control over the air-fuel mixture under any driving conditions. Occasionally, though, buildup from impurities in the fuel, or injector system electrical or mechanical problems may contribute to rough idle, misfires, harmful emissions, poor engine performance, and fuel economy.

This guide can help you get those injectors back in shape applying some simple tests you can apply to both TBI and EFI systems.

Testing TBI Injectors
- Checking Fuel Spray
Testing EFI Injectors
How to Test a Dead Injector
- Checking the Injector Coil
- Checking the Injector Controlling Circuit
Using a Noid Light
Quiz - Test Your Knowledge of EFI Systems
TBI System
TBI System | Source

Testing TBI Injectors

You can visually inspect the fuel spray on TBI system injectors. Fuel flow spray is readily accessible, making it fast to check for potential problems.

Checking Fuel Spray

  1. Remove the cover off the air cleaner housing (where the air filter lives).
  2. Ask an assistant to start (or crank the engine, if it doesn't start).
  3. Check the spray pattern coming our of the injector.

* Fuel should come out partially atomized in an inverted V pattern. A single, solid spray or irregular pattern means the injector needs cleaning, or that an internal part wore out or broke.

You can try to fix an irregular fuel spray pattern by adding a quality cleaning fuel system additive to the fuel tank. Or take your car to the service shop and have them clean the system.

* On the other hand, if you don't see fuel coming out, there may be several reasons for this:

  • Blocked fuel injector
  • Bad injector
  • Injector not receiving power
  • Bad fuel pressure regulator
  • Fuel filter clogged
  • Bad fuel pump

To troubleshoot the injector, apply the tests described in the following section Testing EFI Injectors.

Electronic Fuel Injection system components.
Electronic Fuel Injection system components. | Source

Testing EFI Injectors

Unlike injectors on throttle body fuel injection (TBI) systems, those used on an Electronic Fuel Injection (EFI) configuration don't have their fuel spraying accessible for inspection. And sometimes the fuel rail assembly, which holds the injectors in place, offer little room to access the injectors without removing the complete assembly. So it's hard to check the spraying pattern of each injector without the proper equipment.

However, you can use a couple of tests that can reveal whether one or more injectors are working on your EFI system and whether the fault is within the injector itself or the controlling circuit.

For this test, you'll listen to each injector to determine whether they are working. As the car computer energizes and de-energizes the injector, the valve inside the injector produces a clicking sound as it opens and closes.

  1. To listen to the injector, you can use a mechanic's stethoscope, an inexpensive tool you can buy at most auto parts stores. However, a long standard screwdriver or even a piece of thin hose of the appropriate length will work just as well.
  2. Start the engine and let it idle.
  3. Apply the parking brake and open the hood.
  4. Place the earpiece of the stethoscope on your ears and the tip of the tool against the side of the injector. If you decided to use a long screwdriver, place the tip of the screwdriver against the body of the injector and the end of the tool's handle against your ear.
  5. When the injector opens and closes, you can hear a clicking sound. The sound comes from the solenoid inside the injector activating and deactivating the injector's valve. If you don't hear the clicking sound, either the solenoid has failed or the computer isn't sending the pulse signal. So you have a dead injector.
  6. Repeat this test on each injector and take note of dead injectors, so that you can test them in the following section.

WARNING on Multimeter Use

When troubleshooting electronic components on your car with a digital multimeter, always use a 10 Megohm (minimum) impedance multimeter to prevent circuit damage.

Heated tip injector.
Heated tip injector. | Source

How to Test a Dead Injector

One of the tests you can do on a dead injector is by checking the injector's coil. For this test, you'll need a multimeter and the resistance value for the coil inside the injector. You can look up the injector resistance value in the service manual for your particular vehicle make and model. If necessary, buy an inexpensive, aftermarket repair manual at your local auto parts store or online. And if you haven't used a multimeter before, check the next video for a quick review on how to operate a multimeter.

Checking the Injector Coil

  1. With the engine off, unplug the electrical connector of the injector you need to test.
  2. Set your digital multimeter to an appropriate value on the Ohms scale according to the resistance specifications for your particular fuel injector (usually, you need to set the multimeter to read at least up to 30 Ohms).
  3. Probe the injector electrical terminals—polarity doesn't matter.
  4. A resistance reading other than the specified in your service manual means you need to replace the injector. For example:
  • If your meter reads infinite resistance, it means the coil in the injector is opened.
  • If your reading is jumping all over the place, the coil is partially opened.
  • however, if you read zero resistance, the coil is shorted.

Checking the Injector Controlling Circuit

You can check for power and pulse signal as it comes from the computer on each dead injector using a test light, an inexpensive an efficient tool.

  1. First, hook the test-light clip to a bolt or bare metal bracket on the engine.
  2. Unplug the electrical connector from the fuel injector you want to test.
  3. Turn the ignition switch to the ON position.
  4. Touch the terminals (one at a time) of the harness connector with the test light. One of the terminals should make the test light glow, this is the injector power source coming from the computer. If the test light doesn't glow, you've found the problem. Check the power side of the circuit for a short, blown fuse, or bad connection along the circuit, including the computer.
  5. Now, plug back the harness connector to the fuel injector and hook the test light clip to the positive side of the battery.
  6. Have an assistant crank or start the engine.
  7. Back probe the opposite wire on the fuel injector connector (this is the pulse signal coming from the computer). If you can't back probe the wire, insert a pin through the wire and use the pin to probe the wire.
  8. This time, the test light should flash, meaning the injector is receiving the pulse signal from the computer to open and close the injector.

* If this is a dead injector and the power and the pulse signals are present, replace the injector.

* If the test light remains lit, the device driver in the computer may have failed or there's a problem in the circuit.

Check your repair manual for further tests, if necessary.

Using a Noid Light

You can also test the injector controlling circuit using a noid light. This is the easiest way to test an injector feeding circuit. The noid light connects to the harness of the fuel injector you want to test. You can loan or buy a set of noid lights from your local auto parts store. Just make sure the set you are borrowing or buying includes the noid light you need for your particular vehicle make and model. Follow the instructions that come with the tool.

If any of the previous tests came out negative, it doesn't necessarily mean you injectors are operating correctly. You tested for some common problems you can troubleshoot at home, but one or more injectors may have a worn or dirty (less common) valve, or a weak or broken return spring that's causing the injector to block or leak fuel. Some of these problems can be a challenge to diagnose without the right tools. But a repair auto shop with professional equipment can help you pinpoint these type of problems.

Test Your Knowledge of EFI Systems

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This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

  • What is the cause of an inoperative fuel pump?

    Make sure the pump is receiving adequate voltage. Check for fuel pressure and (if possible) volume. Also, it is possible the pump is worn out and in need of replacement, if voltage/current checks out OK.

  • What is a fuel pressure test?

    This other post gives you the idea about pressure and how to go about the test:

  • What causes inconsistent fuel injector noid readings in different cylinders?

    This could be a problem with the injector drivers, a loose wire in the circuit or problem with the computer itself.

  • what do you repair or replace if there is no pulse on the injectors when every other thing is okay?

    Check the connection at the ECM. If none of the injectors are receiving the signal, there could be the problem. Look up the electric diagram for your model. If you don't have the manual, your local library may have a copy.

  • Do TBI injectors have to be placed facing a certain way?

    Yes, the mounting base will pretty much set it back in place.


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